The direct capture of CO₂ from air (DAC) has been shown a growing interest for the mitigation of greenhouse gases but remains controversial among the engineering community. The high dilution level of CO₂ in air (0.04%) indeed increases the energy requirement and cost of the process compared to carbon capture from flue gases (with CO₂ concentrations around 15% for coal power plants). Until now, solid sorbents (functionalized silica, ion exchange resins, metal–organic frameworks, etc.) have been proposed to achieve DAC, with a few large-scale demonstration units. Gas-liquid absorption in alkaline solutions is also explored. Besides adsorption and absorption, membrane processes are another key gas separation technology but have not been investigated for DAC yet. The objective of this study is to explore the separation performances of a membrane unit for CO₂ capture from air through a generic engineering approach. The role of membrane material performances and the impact of the operating conditions of the process on energy requirement and module production capacity are investigated. Membranes are shown to require a high selectivity in order to achieve purity in no more than two stages. The specific energy requirement is globally higher than that of the adsorption and absorption processes, together with higher productivity levels. Guidelines on the possibilities and limitations of membranes for DAC are finally proposed.

Polarization phenomena play a key role in membrane separation processes but remain largely unexplored for gas separations, where the mass transfer resistance is most often limited to the membrane. This assumption, which is commonly used today for the simulation of membrane gas separations, has to be reconsidered when high-performance materials, showing a very high permeance and/or selectivity, are used. In this study, a series of steady-state separation performances experimentally obtained on CO₂/CH₄ mixtures with a zeolite membrane are compared to the predictions of a dedicated 1D approach, recently derived and validated through CFD simulations. Polarization effects are shown to generate a significant negative impact on the separation performances, both in terms of the productivity and separation efficiency. The 1D model predictions, based on pure gas permeance data and without any adjustable parameters, are in very good agreement with the experimental data. This fast and efficient modeling approach can easily be implemented in simulation or process synthesis programs for the rigorous evaluation of membrane gas separation processes, when high-performance materials are used.

This article investigates the performance of Faradaic electro-swing reactive adsorption (ESA) for CO₂ capture using simulations. Traditional methods such as amine scrubbing face energy efficiency challenges, particularly at low CO₂ concentrations. ESA, which uses electricity for CO₂ regeneration, offers a promising alternative due to its isothermal operation and scalability. The study models ESA using quinone-based redox-active CO₂ carriers in an electrochemical cell with an ionic liquid electrolyte, allowing reversible adsorption and release through voltage control. The model estimates system productivity and energy consumption, considering transport and chemical kinetics. Key findings show that operating parameters, such as applied potential and gas flow rate, have a significant effect on efficiency. Applying a potential of −1.3 V improved the adsorption capacity, reducing CO₂ capture time compared to −1.1 V. At a 1% CO₂ concentration and a low flow rate, effective capture resulted in a productivity of 1.6 kg/(m³·day) with an energy consumption of 0.6 MWh/tCO₂. However, higher gas flow rates reduced capture efficiency due to CO₂ transport limitations in the ionic liquid. Optimization of electrode design is essential to improve ESA efficiency.